Slacking guitar strings for flying isn't about baggage handling (though it would help), i'ts about temperature. Aeroplane holds tend to be unheated, so the strings will try and shrink. Since the guitar stays the same size, they can't shrink, the tension goes up, strings break or guitar bends. That's the usual argument.
On the other hand, if the guitar has a steel truss rod, I suspect the neck length would also shrink , so you wouldn't need to slack the strings. Mind you I've never heard anyone recommend slacking the truss rod before flying.
Any luthiers out there who can advise on this one? ( Other than the obvious advice of not putting guitars in aeroplane holds! )